This invention relates to the transport of tape between reels, and, more particularly, to tension control of tape during direct transport between the reels without significant buffering of the tape.
A tape that is directly transported between a supply reel and a take-up reel requires precise control of the tape tension to provide smooth constant tape speed, and, if the tape is to be accelerated and decelerated, to provide smooth acceleration and deceleration of the tape, and to prevent damage to the tape. One example of a direct tape transport is a magnetic tape drive, in which a magnetic tape is transported along a tape path past a read/write head located in the tape path, such that the magnetic read/write head may read and/or write data on the magnetic tape, and the magnetic tape is stopped and started to read and/or write data at desired locations of the magnetic tape. Another example of a direct tape transport is a printing press in which a tape (paper web) is transported between reels past printing rolls under precise timing, and new rolls of paper web must be accelerated to speed smoothly without damage and without smudging at the printing rolls. Herein, the term xe2x80x9ctapexe2x80x9d is defined as comprising tape or web in any suitable elongate form; the term xe2x80x9creelxe2x80x9d is defined as comprising a reel or spool that is removable or permanent, and provides the spool or axis at which the tape is wound and/or unwound; and the terms xe2x80x9csupplyxe2x80x9d and xe2x80x9ctake-upxe2x80x9d reels refer to the two reels of a two reel system, typically respectively from which the tape originates and to which the tape is directed, but the tape may be wound and unwound with respect to either reel in a bi-directional fashion.
One or more electromechanical tape tension transducers can be mounted in the tape path to monitor tape tension, and the supply and/or take-up reel motors are controlled to provide a nominal tape tension and to tend to offset errors in tape tension. Examples of tape tension transducers comprise U.S. Pat. Nos. 5,282,586, 5,277,378, 5,039,027, 4,807,107, and 3,606,201, all of which illustrate the use of tension arms. U.S. Pat. No. 3,809,335 indicates that other types of sensors may also be used, such as a pressure responsive air jet or bearing, or a load cell. Another example of a tape tension transducer is a direct sensing pressure transducer. However, a delicate balance is maintained in designing tape tension transducers to both be robust and reliable, yet also to be accurate throughout a wide range of frequencies of tape tension variation, without resonances. The tape tension transducers, by virtue of the mechanical aspects, have upper limits to the effective range of tension measurement, for example, defined by resonance of the mechanism. U.S. Pat. No. 4,400,745 estimates tension by summing the currents from both the supply reel and the take-up reel motors, divides the result by two, and equates the same to tape tension. In U.S. Pat. No. 3,913,866, a signal proportional to the angular velocity of the supply and take-up reels is generated at each reel and supplied to a torque device at the other reel. Japan JP6-349153 compares a frequency of a motor or reel to an expected frequency to correct the torque of a motor to obtain stable tape tension without providing a tension lever or arm. Such rough estimates of the tape tension are not sufficiently accurate for modern high speed tape motion.
A precise control of tape tension is illustrated by U.S. Pat. No. 4,015,799 which determines tape tension based on the differences in torque as applied to the supply and take-up reels. Lineal tape position and angular reel displacement for both reels are monitored to determine reel radii, tape inertia and velocity or position error. A motor current algorithm is utilized to generate the appropriate torque for each reel to drive the error to zero along a predetermined profile with negligible tape tension disturbances. U.S. Pat. No. 5,860,610 discusses defining the reel inertia similar to that of the ""799 patent, employing a conversion table, to control the torque of a motor, and a second embodiment in which the output of a pressure sensitive tension sensor is differentiated and combined with the output of a torque modulator. In a third embodiment, change in rotating speed of a supply reel is employed with the torque modulator in an attempt to suppress the change in speed.
Modern tapes are driven at high speeds and are more sensitive to changes in tape tension, for example, in that the speed variation at the tape head will result in data read or write errors, or smudging at a print roll. In the case of magnetic tape, modern tapes are thinner to allow more tape to be spooled on a reel at the same diameter, and thereby to allow a greater data storage capacity. Hence, such tapes are more sensitive to changes in tape tension, and may be subjected to damage, such as tape stretch.
In accordance with the present invention, tension control systems, methods, and tape transports provide dynamic tension control for tape transported along a tape path between a supply reel and a take-up reel, the supply reel driven by a supply reel motor, and the take-up reel driven by a take-up reel motor.
In one embodiment, a supply tachometer measures the rotational angular displacement of the supply reel, and a take-up tachometer measures the rotational angular displacement of the take-up reel. A tension control system controller:
measures the rotational angular displacement of the supply reel from the supply tachometer;
measures the rotational angular displacement of the take-up reel from the take-up tachometer;
determines from the measured rotational angular displacement of the supply reel, the rotational angular velocity of the supply reel;
determines from the determined rotational angular velocity of the supply reel, a linear speed for the tape at the supply reel;
determines from the measured rotational angular displacement of the take-up reel, the rotational angular velocity of the take-up reel;
determines from the determined rotational angular velocity of the take-up reel, a linear speed for the tape at the take-up reel;
compares the determined linear speed for the tape at the supply reel to the determined linear speed for the tape at the take-up reel, to determine a delta velocity between the linear speeds; and
operates at least one of the supply reel motor and the take-up reel motor in accordance with a function of the delta velocity to provide a torque to at least one of the supply reel and the take-up reel tending to reduce the delta velocity.
In a further embodiment, the controller operates the reel(s) in accordance with a function of the delta velocity, of the current inertial characteristics of the reel(s), and of the current radius of tape of the reel(s).
In a still further embodiment, the controller determines the linear speed for the tape at each of the supply reel and the take-up reel by determining the linear speed from the rotational angular velocity of the reel as a function of the current radius of tape of the reel.
In another embodiment, the controller operates the supply reel motor and the take-up reel motor in accordance with a function of the delta velocity to provide equal and opposite torques to the supply reel and the take-up reel tending to reduce the delta velocity.
In still another embodiment, a tape tension transducer is mounted in the tape path for measuring the tension for the tape in the tape path; and a controller:
measures the tension for the tape in the tape path from the tape tension transducer;
determines whether the measured tension is within a predetermined range of a nominal tension;
and, if the measured tension is within the predetermined range of the nominal tension,
measures the rotational angular displacement of the supply reel from the supply tachometer;
measures the rotational angular displacement of the take-up reel from the take-up tachometer;
determines from the measured rotational angular displacement of the supply reel, the rotational angular velocity of the supply reel;
determines from the determined rotational angular velocity of the supply reel, a linear speed for the tape at the supply reel;
determines from the measured rotational angular displacement of the take-up reel, the rotational angular velocity of the take-up reel;
determines from the determined rotational angular velocity of the take-up reel, a linear speed for the tape at the take-up reel;
compares the determined linear speed for the tape at the supply reel to the determined linear speed for the tape at the take-up reel, to determine a delta velocity between the linear speeds; and
operates at least one of the supply reel motor and the take-up reel motor in accordance with a function of the delta velocity to provide a torque to at least one of the supply reel and the take-up reel tending to reduce the delta velocity.
In an additional embodiment, a controller:
A) establishes a nominal tension on the tape:
1) measures the rotational angular displacement of the supply reel from the supply tachometer;
2) derives reel radii and tape inertial values corresponding thereto;
3) measures the rotational angular displacement of the take-up reel from the take-up tachometer;
4) derives reel radii and tape inertial values corresponding thereto; and
5) operates the supply reel motor and the take-up reel motor to apply a nominal tension to the tape; and
B) maintains tension control tending to maintain the tape tension constant:
1) determines from the measured rotational angular displacement of the supply reel, the rotational angular velocity of the supply reel;
2) determines from the determined rotational angular velocity and derived reel radii values of the supply reel, a linear speed for the tape at the supply reel;
3) determines from the measured rotational angular displacement of the take-up reel, the rotational angular velocity of the take-up reel;
4) determines from the determined rotational angular velocity and derived reel radii values of the take-up reel, a linear speed for the tape at the take-up reel;
5) compares the determined linear speed for the tape at the supply reel to the determined linear speed for the tape at the take-up reel, to determine a delta velocity between the linear speeds; and
6) operates at least one of the supply reel motor and the take-up reel motor in accordance with a function of the determined delta velocity and derived inertial values to provide a torque to at least one of the supply reel and the take-up reel tending to reduce the delta velocity.